Although polyolefin resins have long found utility in food packaging and food storage container applications, a polyolefin composition with the desired balance of properties in the form of a film, coating or molding has not been available to fabricators and pakagers. For example, an optimum polyolefin resin composition for use as a sealant layer in packaging and storage applications would possess a number of key performance properties such as low heat seal and hot tack initiation temperatures, a high hot tack strength, a broad hot tack sealing window, good interlayer adhesion, a high softening point and low hexane extractables levels.
The commercial importance of balanced sealant properties is well understood. That is, low heat seal and hot tack initiation temperatures are important for improved sealing speeds and reduced energy utilization. A broad hot tack sealing window at high hot tack strengths (i.e., the seal temperature range where the hot tack strength is greater than or equal to about 46 g/cm as measured by the Dupont spring method or gl eater than or equal to about 3.31 Newton/15 mm (5.6 N/in.) as measured using a mechanical hot tack tester such as, for example, a Top Wave Sealing unit) is important for insuring package integrity, sealing equipment flexibility and low package leaker rates. Good interlayer adhesion is also important for good package integrity as well as good package or container aesthetics. High softening points or temperatures are desired where goods are packaged at elevated temperatures such as in hot-fill applications. Low hexane extractables are required for food contact applications.
Traditionally, when attempting to achieve balanced sealant properties, enhancement of one particular resin property has required some sacrifice with respect to another important property. For instance, with ethylene alpha-olefin polymers, low heat seal and hot tack initiation temperatures are typically achieved by increasing the comonomer content of the resin. Conversely, high Vicat softening points and low levels of n-hexane extractives are typically achieved by decreasing the comonomer content of the resin. Accordingly, improving the resin with respect to seal initiation typ ically results in proportionally reduced Vicat softening temperature and proportionally increased extractable level.
Several important multilayer packaging and storage structures consisting of a polypropylene layer, particularly, a biaxially oriented polypropylene homopolymer (BOPP) base or core layer. Typically, BOPP structures utilize polypropylene copolymers and terpolymers as sealant materials (and/or adhesive layers) to insure good interlayer adhesion to the BOPP base layer. While polypropylene copolymers and terpolymers do indeed provide good interlayer adhesion to BOPP base layers as well as good hot tack strength performance, these copolymers and terpolymers invariably exhibit undesirably high heat seal and hot tack initiation temperatures.
Other polyolefin materials have also been used as sealant materials for multilayer packaging and storage structures. However, in general, known polyolefin sealant materials do not provide the desired overall property balance and/or process flexibility desired by converters and packagers. For example, TAFMER.TM. resins (supplied by Mitsui Petrochemical) are known to provide sealants with relatively low seal initiation temperatures, however, TAFMER.TM. resins are not known to provide the overall desired performance balance (either as single component sealants nor when used as polymer blend component materials). Also, TAFMER.TM. resins are considered to be relatively expensive and are continually in limited commercial supply.
Relative to TAFMER.TM. resins, heterogeneously branched ethylene polymers such as linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE) are readily available. However, similar to TAFMER.TM. resins, in general, heterogeneously branched ethylene polymers do not provide the desired overall property balance for optimum use as sealant materials and they are particularly ill-suited for BOPP structures. For example, heterogeneously branched linear low density polyethylene (LLDPE) (and, as such, sealant layers made from these polymers) are particularly deficient in regards to interlayer adhesion to polypropylene layers.
Homogeneously branched ethylene polymers such as AFFINITY.TM. resins supplied by The Dow Chemical Company are also available for use as sealant materials. While homogeneously branched ethylene polymer materials generally exhibit improved sealing initiation performance, the hot strength of these resins is invariably compromised to meet Theological requirements for coextrusions and thereby insure good interfacial stability.
U.S. Pat. No. 4,429,079 to Shibata, et al. discloses an ethylene/alpha-olefin copolymer blend composition comprising a mixture of (A) 95-40 weight percent of a random copolymer of ethylene and an alpha-olefin having 5 to 10 carbon atoms which has a melt index of 0.1 to 20 g/10 min., a density of 0.910 to 0.940 g/cc, a crystallinity by X-rays of 40 to 70%, a melting point of 115 to 130.degree. C., and an ethylene content of 94 to 99.5 mol %; and (B) 5 to 60% by weight of a random copolymer of ethylene and an alpha-olefin having 3 to 10 carbon atoms which has a melt index of 0.1 to 50 g/10 min., a density of 0.870 to 0.900 g/cc, a crystallinity by X-rays of 5 to 40%, a melting point of 40 to 100.degree. C. and an ethylene content of 85 to 95 mol %. The (A) component polymer is said to be produced by a titanium catalyst system and the (B) component polymer is said to be produced by a vanadium catalyst. Both of these catalyst systems are known as Ziegler-Natta type catalysts which produce linear ethylene alpha-olefin polymers. That is, the polymer will have a linear molecular backbone without any long chain branching. Further, the (A) component polymer will also have a heterogeneously branched short chain distribution, while the (B) component polymer will have a homogeneously branched short chain distribution. The film fabricated from the Shibata et al. composition allegedly has good low-temperature heat sealability, heat seal strength, pin hole resistance, transparency and impact strength, making such film suitable for premium packaging applications. However, Shibata et al. do not disclose films with high ultimate hot tack strengths (i.e., values.gtoreq.3.31 N/mm) and analysis of the data disclosed in the Examples provided by Shibataet al. reveals the properties of such film, particularly heat sealability, are additive and vary linearly with respect to the densities of blended component polmer.
U.S. Pat. No. 4,981,760 to Naito et al. discloses a polyethylene mixture having a density of from 0.900 to 0.930 g/cc and melt flow rate of from 0.1 to 100 g/10 in., which comprises (I) from 60 to 99 parts by weight of an ethylene-.alpha.-olefin random copolymer comprising ethylene and an .alpha.-olefin having from 4 to 10 carbon atoms, the copolymer having an .alpha.-olefin content of from 2.0 to 10 mol % and a density of from 0.895 to 0.915 g/cc, the programmed-temperature thermogram of said copolymer as determined with a differential scanning calorimeter after being completely melted and then gradually cooled showing an endothermic peak in a range of from 75.degree. to 100.degree. C., with the ratio of an endotherm at said peak to the total endotherm being at least 0.8, and (II) from 1 to 40 parts by weight of high-density polyethylene having a density of at least 0.945 g/cc, the programmed-temperature thermogram of said high-density polyethylene as determined with a differential scanning calorimeter after being completely melted and allowed to cool showing an endothermic peak at 125.degree. C., or higher, wherein the sum of (I) and (II) amounts to 100 parts by weight. The component polymer (I) is said to be manufactured using a vanadium catalyst and the film allegedly has improved heat sealability and hot tack. Naito et al. do not disclose fabricated films comprising a component polymer (II) with a density less than 0.945 g/cc. Also, Naito et al. describe a film having low heat seal or hot tack initiation temperatures when the lower density component polymer (I) concentration is fairly high (i.e.,.gtoreq.85 parts) which is expected to result in lower Vicat softening points.
U.S. Pat. No. 5,206,075 to Hodgson et al. discloses a multilayer heat sealable film comprising a base layer and a heat sealable layer superimposed on one or both sides of the base layer. As the base layer, Hodgson discloses a blend of: (a) an olefin polymer having a density greater than 0.915 g/cc; and (b) a copolymer of ethylene and a C.sub.3 -C.sub.20 alpha-monoolefin, with the copolymer (b) having a density of from about 0.88 to about 0.915 g/cc, a melt index of from about 0.5 to about 7.5 dg/min, a molecular weight distribution of no greater than about 3.5, and a composition distribution breadth index greater than about 70 percent. As the heat sealable layer, Hodgson discloses a layer comprising a copolymer as defined in (b) with respect to the base layer. Hodg son does not disclose the use of a blend, such as that employed in the base layer (a), as a suitable sealing layer and the preferred olefin polymer for component (a) of the base layer is a copolymer of propylene with about 1-10 mole percent ethylene. As such, this disclosure appears to generally indicate that good interlayer adhesion requires that the base layer and sealant layer have similar olefin chemistries.
The compositions disclosed by Shibata et al., Naito et al. and Hodgson et al. and other known seal ant materials are deficient in one respect or another. These materials do not provide the balanced sealant properties desired by converters and packagers. In particular, known materials are not particularly well-suited for use as sealant materials in BOPP structures. As such, there is a need for polymer compositions characterized by good interlayer adhesion to polypropylene, low heat seal and hot tack imatiation temperatures, high hot tack strength and a broad high hot tack sealing window There is also a need for polymer sealant compositions which have low levels of n-hexane extractives, i.e., less than 15 weight percent, preferably less than 10 weight percent, more preferably less than 6 weight percent, most preferably less than 3 weight percent, as such compositions would be useful in direct food contact applications.